For a liquid developer, in general, those with a form that colored resin particles containing coloring agents such as pigments are dispersed in an electrically insulating dispersion medium are used. As one of methods for producing the liquid developer, there is a coacervation method. The coacervation method is a method of precipitating a resin contained in a mixed solution in a dissolved state in a manner of encapsulating a coloring agent by removing a solvent from the mixed solution containing the solvent dissolving the resin and an electrically insulating dispersion medium which does not dissolve the resin; and dispersing the colored resin particles in the electrically insulating dispersion medium.
A liquid developer obtained by the method is supposed to have a good electrophoretic property since the colored resin particles have shapes closer to spheres and more uniform particle sizes than those obtained by a wet-milling method.
However, the coacervation method has a problem that the colored resin particles are easily agglomerated at the time of precipitation of the resin and due to the coarsening of the particles, the dispersion stability and optical characteristics of the obtained developer are insufficient.
Therefore, to solve the above-mentioned problem, there is proposed a method using a polymer compound having an acid group and a polymer compound having a basic group in the coacervation method. In this method, first, either one of the polymer compounds is adsorbed on the coloring agent surface. Next, the coloring agent is encapsulated in the other polymer compound, so that the colored resin particles are stably dispersed in an electrically insulating dispersion medium (e.g., refer to Patent Document 1).
However, the above-mentioned method is a technique based on the concept of the conventionally well known acid-base interaction which aims to achieve dispersion stability by treating the pigment surface with a compound containing an acid group or a basic group and increasing the affinity to the resin having the other group. Further, the disclosed compounds are fairly commonly used acrylic resins and styrene-acrylic resins. Since these compounds are not materials that are designed focusing on the dispersibility of particles, an effect of sufficiently improving the dispersibility of fine colored resin particles cannot be expected. Moreover, in terms of the suitability for a liquid developer, there is another problem that the charging property and the electrophoretic property of the colored resin particles are deteriorated and thus it results in impossibility of obtaining a good image quality.    Patent Document 1: Japanese Kokai Publication 2001-31900